1. Field of the Invention
This invention relates to the preparation of alkylalkoxysilanes having polysulfide bridges wherein there are more than 2 sulfur atoms in the molecule. This invention is particularly directed to the preparation of bis-(alkoxylalkylsilyl)-polysulfides particularly bis-[.gamma.-(trialkoxysilyl)-alkyl] polysulfides of at least 3 sulfur atoms in the compound.
2. Discussion of the Prior Art
The preparation of sulfurous silanes of the type named is already known. These compounds are useful as adhesivizers in sulfur-vulcanizable-rubber mixtures reinforced with organic materials such as glass, silicon dioxide and the like.
Methods for the preparation of these compounds are also known. One such method involves reacting a mercaptosilane with a sulfur chloride or with the oxidants bromine, iodine, thionyl chloride or dialkyl sulfoxides which are known agents for the dehydrogenation of mercaptans to disulfides. Such a process is described in Houben-weyl, Houben-Weyl, Methoden der organischen Chemie", 4th Ed., 1959, vol. 9, pages 59-65. However, such a process for the preparation of sulfurous silanes suffers from the disadvantage that the sulfur chlorides employed are sensitive to moisture, decompose readily and especially in the case of S.sub.2 Cl.sub.2, have an unpleasant lachrymatory odor. When dialkyl sulfoxides are used it is also disadvantageous in that water is formed during the reaction. This so formed water reacts with the alkoxy group of the silane resulting in hydrolysis thereof and simultaneous formation of siloxanes of higher molecular weight.
Another method for the preparation of these compounds utilizes chloralkylsilanes which are reacted with alkali sulfides or polysulfides. Such a process is described in German Offenlegungsschrift No. 2,141,159. The disadvantage of this method lies in the fact that it is necessary to separate the alkali chloride that forms and consequently an additional step is involved in the process with its corresponding investment in time and apparatus.
All known methods are, furthermore, encumbered by the disadvantage that they are best performed in a solvent, preferably at elevated temperature. Consequently for the isolation of the product, the solvent must be removed by an additional separation procedure.
It has, therefore, become desirable to provide a process for the preparation of alkylalkoxysilanes having polysulfide bridges which process can advantageously be carried out in the absence of a solvent, does not yield water which will interfere with the alkoxy group on the silane, does not utilize a reactant which can easily decompose and does not involve the creation of unpleasant, noxious or lachrymatory odors.